Recent African origin of modern humans

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This article is about modern humans. For migrations of early humans, see Out of Africa I.
Map of the migration of modern humans out of Africa, based on mitochondrial DNA. Colored rings indicate thousand years before present.

In paleoanthropology, the recent African origin of modern humans, also called the "Out of Africa" theory (OOA), recent single-origin hypothesis (RSOH), replacement hypothesis or recent African origin model (RAO), is the dominant model of the geographic origin and early migration of anatomically modern humans (Homo sapiens). According to this model, modern humans originated in Africa and started to disperse through the world roughly 50,000 to 100,000 years ago.

Recent single origin of modern humans in East Africa was cited as the scientific consensus as of the mid-2000s.[1][2] The major competing hypothesis of "recent single origin" has been the multiregional origin of modern humans, which envisions a wave of Homo sapiens migrating earlier from Africa and interbreeding with local Homo erectus populations in multiple regions of the globe.[3][4] In the 2010s, the discovery of evidence of archaic admixture in populations outside of Africa has complicated the picture.[5]

As of 2011, it appears likely that there were two waves of migration out of Africa, the first taking place between 130,000–115,000 years ago via northern Africa,[6][7][8][9] which appears to have mostly died out or retreated (although there is some evidence of a presence of modern humans in China about 80,000 years ago,[10] and a second via the so-called Southern Route, following the southern coastline of Asia, which led to the lasting colonization of Eurasia and Australia by around 50,000 years ago. Europe was populated by an early offshoot which settled the Near East and Europe (post-Toba hypothesis).[11][12]

African origins of Homo sapiens[edit]

Development[edit]

Anatomical comparison of the skulls of a modern human (left) and Homo neanderthalensis (right).

Anatomically modern humans originated in Africa by about 200,000 years ago. The trend in cranial expansion and the acheulean elaboration of stone tool technologies which occurred between 400,000 years ago and the second interglacial period in the Middle Pleistocene (around 250,000 years ago) provide evidence for a transition from Homo erectus to H. sapiens.[13] In the Recent African Origin (RAO) scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus.

Homo sapiens idaltu, found at site Middle Awash in Ethiopia, lived about 160,000 years ago.[14]

Beginning about 100,000 years ago, evidence of more sophisticated technology and artwork begins to emerge and by 50,000 years ago, fully modern behaviour becomes more prominent. Stone tools show regular patterns that are reproduced or duplicated with more precision while tools made of bone and antler appear for the first time.[15][16]

Fossil finds outside Africa[edit]

Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated to 80,000 to 100,000 years ago.[17] A fossil of a modern human dated to 54,700 years ago was found in Manot Cave in Israel, named Manot 1,[18][19] though the dating was questioned by Groucutt et al. (2015).[20] Fossils from Lake Mungo, Australia, have been dated to about 42,000 years ago.[21][22] The Tianyuan cave remains in China's Liujiang region have a probable date range between 38,000 and 42,000 years ago. The Tianyuan specimens are most similar in morphology to Minatogawa Man, modern humans dated between 17,000 and 19,000 years ago and found on Okinawa Island, Japan.[23][24]

Dispersals out of Africa[edit]

Map of early human migrations[25]
1. Homo sapiens
2. Neanderthals
3. Early hominins

Anatomically modern humans originated in Africa by about 200,000 years ago. The accepted theory is that there were two dispersals from African populations,[11][26] an early one via northern Africa which left traces in some human populations,[27] and a second one which populated the entire world.

The first dispersal took place between 130,000–115,000 years ago via northern Africa, but died out or retreated.[6][7][8][9] Chinese researchers question this extinction, claiming that modern humans were present in China already 80,000 years ago.[10]

A second dispersal took place via the so-called Southern Route, either before[28] or after[11][12] the Toba event, which happened between 69,000 and 77,000 years ago.[28] This dispersal followed the southern coastline of Asia, crossing about 250 kilometres (155 mi) of sea, and colonized Australia by around 50,000 years ago. According to this theory, Europe was populated either by a migration out of India, which was repopulated from southeast Asia after the Toba-event (pre-Toba hypothesis), or by an early offshoot which settled the Near East and Europe (post-Toba hypothesis).[11][12]

Early northern Africa dispersal[edit]

The early northern Africa dispersal took place between 130,000–115,000 years ago. The discovery of stone tools in the United Arab Emirates in 2011 indicated the presence of modern humans between 100,000 and 125,000 years ago,[6] leading to a resurgence of the "long-neglected" North African route.[7][29][8][9]

Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated to 80,000 to 100,000 years ago. These humans seem to have either become extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by southbound Neanderthals escaping the colder regions of ice age Europe.[30] Hua Liu et al. analyzed autosomal microsatellite markers dating to about 56,000 years ago. They interpret the paleontological fossil as an isolated early offshoot that retracted back to Africa.[17]

According to Kuhlwilm et al. (2016), Neanderthals received gene flow from modern humans around 100,000 years ago, from humans which split off from other modern humans around 200,000 years ago.[31][32] They asserted that "the ancestors of Neanderthals from the Altai Mountains and early modern humans met and interbred, possibly in the Near East, many thousands of years earlier than previously thought."[31] According to co-author Ilan Gronau, "This actually complements archaeological evidence of the presence of early modern humans out of Africa around and before 100 ka by providing the first genetic evidence of such populations."[32]

Chinese research questions the extinction of this early dispersal. Shen et al. (2002) have dated Liujang Man to 111,000 to 139,000 years before the present.[33] Liu et al. (2015) claim that modern human teeth have been found in China dating to at least 80,000 years ago.[10]

Southern Route dispersal[edit]

Red Sea crossing
Main article: Coastal migration

Coastal route[edit]

By some 70,000 years ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East. It has been estimated that from a population of 2,000 to 5,000 individuals in Africa,[34] only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea.[35]

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Persia until reaching India, which appears to be the first major settling point.[36] Spencer Wells in The Journey of Man (2002) argued for the route along the southern coastline of Asia, across about 250 kilometres (155 mi), reaching Australia by around 50,000 years ago.

Today at the Bab-el-Mandeb straits, the Red Sea is about 20 kilometres (12 mi) wide, but 50,000 years ago sea levels were 70 m (230 ft) lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, they were narrow enough and there may have been islands in between to have enabled crossing using simple rafts.[37][38] Shell middens 125,000 years old have been found in Eritrea,[39] indicating the diet of early humans included seafood obtained by beachcombing.

Dating: pre-or post-Toba[edit]

The dating of the Southern Dispersal is a matter of dispute.[28] It may have happened either pre- or post-Toba, a catastrophic volcanic eruption that took place between 69,000 and 77,000 years ago at the site of present-day Lake Toba. Stone tools discovered below the layers of ash disposed in India may point to a pre-Toba dispersal, but the exact source of these tools is disputed.[28]

An indication for post-Toba is haplo-group L3, that originated before the dispersal of humans out of Africa, and can be dated to 60,000–70,000 years ago, "suggesting that humanity left Africa a few thousand years after Toba."[28] Yet, new research showing slower than previously thought genetic mutations in human DNA was published in 2012, indicating a revised dating for the migration of between 90,000 and 130,000 years ago.[40]

Peopling of Europe[edit]

Pre-Toba[edit]

According to Martin Meredith, members of one branch of Homo sapiens left Africa at some point between 125,000 and 60,000 years ago, and over time these humans replaced other populations of the genus Homo such as Neanderthals and Homo erectus.[41]

Post-Toba[edit]

According to Macaulay et al. (2005) an early offshoot from the southern disposal with haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai.[11] This group then branched in several directions, some moving into Europe and others heading east into Asia.[11] This hypothesis is supported by the relatively late date of the arrival of modern humans in Europe as well as by both archaeological and DNA evidence.[11]

Based on an analysis of 55 complete human mitochondrial genomes (mtDNAs) of hunter-gatherers, Posth et al. (2016) argue for a "rapid single dispersal of all non-Africans less than 55,000 years ago."[12]

Multiple dispersals[edit]

Several authors have argued for multiple dispersals.[12][20] Spencer Wells, in The Journey of Man (2002) describes two waves of migration, first the southern coastal route, then a northern migration into Europe[42] at circa 45,000 years ago.[note 1] This possibility is ruled out by Macauly et al. (2005)[44] and Posth et al. (2016),[12] arguing for a single coastal dispersal.

Genetic reconstruction[edit]

Mitochondrial haplogroups[edit]

Map of early diversification of modern humans according to mitochondrial population genetics (see: Haplogroup L).

Within Africa[edit]

The first lineage to branch off from Mitochondrial Eve is L0. This haplogroup is found in high proportions among the San of Southern Africa, the Sandawe of East Africa. It is also found among the Mbuti people.[45][46]

These groups branched off early in human history and have remained relatively genetically isolated since then. Haplogroups L1, L2 and L3 are descendants of L1-6 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3. L3 is about 84,000 years old, and haplogroup M and N are almost identical in age at about 63,000 years old.[47] The relationship between such gene trees and demographic history is still debated when applied to dispersals.[20]

Of all the lineages present in Africa, only the female descendants of one lineage, mtDNA haplogroup L3, are found outside Africa. If there had been several migrations, one would expect descendants of more than one lineage to be found outside Africa. L3's female descendants, the M and N haplogroup lineages, are found in very low frequencies in Africa (although haplogroup M1 populations are very ancient and diversified in North and Northeast Africa) and appear to be more recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and became the dominant haplogroups after the exodus from Africa through the founder effect. Alternatively, the mutations may have arisen shortly after the exodus from Africa.

Southern Route and haplogroups M and N[edit]

Results from mtDNA collected from aboriginal Malaysians called Orang Asli and the creation of a phylogentic tree indicate that the hapologroups M and N share characteristics with original African groups from approximately 85,000 years ago and share characteristics with sub-haplogroups among coastal southeast Asian regions, such as Australasia, the Indian Subcontinent, and throughout continental Asia, which had dispersed and separated from its African origins approximately 65,000 years ago. This southern coastal dispersion would have occurred before the dispersion through the Levant approximately 45,000 years ago.[11] This hypothesis attempts to explain why haplogroup N is predominant in Europe and why haplogroup M is absent in Europe. Evidence of the coastal migration is hypothesized to have been destroyed by the rise in sea levels during the Holocene epoch.[48][49] Alternatively, a small European founder population that initially expressed both haplogroup M and N could have lost haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Persia until reaching India, which appears to be the first major settling point.[36] Haplogroup M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred.[36] Sixty percent of the Indian population belong to Haplogroup M.

The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from mainland Asia. They are evidence of the coastal route of early settlers that extends from India along the coasts of Thailand and Indonesia all the way to Papua New Guinea. Since M is found in high frequencies in highlanders from New Guinea as well, and both the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists think they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration.

From Arabia to India the proportion of haplogroup M increases eastwards: in eastern India, M outnumbers N by a ratio of 3:1. However, crossing over into East Asia, haplogroup N reappears as the dominant lineage. M is predominant in South East Asia but amongst Indigenous Australians N reemerges as the more common lineage. This discontinuous distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.[50]

Autosomal DNA[edit]

A 2002 study of African, European, and Asian populations found greater genetic diversity among Africans than among Eurasians, and that genetic diversity among Eurasians is largely a subset of that among Africans, supporting the out of Africa model.[51]

Conversely, a large-scale study by Coop et al. (2009) found evidence for natural selection in autosomal DNA outside of Africa. The study distinguishes non-African sweeps (notably KITLG variants associated with skin color), West-Eurasian sweeps (SLC24A5) and East-Asian sweeps (MC1R, relevant to skin color). Based on this evidence, the study concluded that human populations encountered novel selective pressures as they expanded out of Africa.[52]

MC1R and its relation to skin color had already been discussed by Harding et al. (2000).[53] According to this study, Papua New Guineans continued to be exposed to selection for dark skin color so that, although these groups are distinct from Africans at other loci, the allele for dark skin color shared by contemporary Africans, Andamanese and New Guineans is an archaism. By contrast, Endicott et al. (2003) suggest convergent evolution.[54]

Pathogenic load[edit]

Another promising route towards reconstructing human genetic genealogy is via the JC virus (JCV), a type of human polyomavirus which is carried by 70-90% of humans and which is usually transmitted vertically, from parents to offspring, suggesting codivergence with human populations. For this reason, JCV has been used as a genetic marker for human evolution and migration.[55]

However, this method does not appear to be reliable for the migration out of Africa, as in contrast to human genetics, JCV strains associated with African populations are not basal. From this Shackelton et al. (2006) conclude that either a basal African strain of JCV has become extinct, or that the original infection with JCV post-dates the migration from Africa. [56]

Admixture of archaic and modern humans[edit]

Evidence for archaic human species (descended from Homo heidelbergensis) having interbred with modern humans outside of Africa was discovered in the 2010s. This concerns primarily Neanderthal admixture in all modern populations except for Sub-Saharan Africans, but evidence has also been presented for Denisova hominin admixture in Australasia (i.e. in Melanesians, Aboriginal Australians and some Negritos).[57] The rate of admixture has been found to be relatively low (1–10%), and the result is still open to revision, as the presence of archaic markers has also been suggested as being due to shared ancestral traits rather than interbreeding.[58]

In 2011, evidence was presented that for archaic admixture in some Sub-Saharan African populations (Biaka Pygmies and San), derived from archaic hominins that broke away from the modern human lineage around 700,000 years.[59] After a survey for the introgressive haplotypes across Sub-Saharan populations, it was suggested that the admixture event happened with archaic hominins that possibly once inhabited Central Africa.[59]

Stone tools[edit]

Further information: Aterian, Baradostian, and Microlith

In addition to genetic analysis, Petraglia et al. also examines the small stone tools (microlithic materials) from Indian subcontinent and explains the expansion of population based on the reconstruction of paleoenvironment. He proposed that the stone tools could be traced back to 35 ka in South Asia, and the new technology might be influenced by environmental change and population pressure.[60]

History of the theory[edit]

Further information: Timeline of human evolution

Classical paleoanthropology[edit]

The frontispiece to Huxley's Evidence as to Man's Place in Nature (1863): the image compares the skeletons of apes to humans.

The cladistic relationship of humans with the African apes was suggested by Charles Darwin after studying the behaviour of African apes, one of which was displayed at the London Zoo.[61] The anatomist Thomas Huxley had also supported the hypothesis and suggested that African apes have a close evolutionary relationship with humans.[62] These views were however opposed by the German biologist Ernst Haeckel, who was a proponent of the Out of Asia theory. Haeckel argued that humans were more closely related to the primates of Southeast Asia and rejected Darwin’s hypothesis of Africa.[63][64]

In the Descent of Man, Darwin speculated that humans had descended from apes which still had small brains but walked upright, freeing their hands for uses which favoured intelligence. Further, he thought such apes were African:[65]

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

— Charles Darwin, Descent of Man[66]

The prediction was insightful, because in 1871 there were hardly any human fossils of ancient hominins available. Almost fifty years later, Darwin's speculation was supported when anthropologists began finding numerous fossils of ancient small-brained hominins in several areas of Africa (list of hominina fossils).

The hypothesis of recent (as opposed to archaic) African origin develops in the 20th century. The "Recent African origin" of modern humans means "single origin" (monogenism) and has been used in various contexts as an antonym to polygenism. The debate in anthropology had swung in favour of monogenism by the mid-20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who hypothesized as late as 1962 that H. sapiens arose five times from H. erectus in five places.[67]

Multiregional origin hypothesis[edit]

The historical alternative to the recent origin model is the multiregional origin of modern humans, initially proposed by Milford Wolpoff in the 1980s.

This view proposes that the derivation of anatomically modern human populations from H. erectus at the beginning of the Pleistocene 1.8 million years BP has taken place within a single, continuous worldwide population. The hypothesis necessarily rejects the assumption of an infertility barrier between ancient Eurasian and African populations of Homo.

The hypothesis was controversially debated during the late 1980s and the 1990s.[68] The now-current terminology of "recent-origin" and "Out of Africa" became current in the context of this debate, over the course of the 1990s.[69]

Originally seen as an antithetical alternative to the recent origin model, the multiregional hypothesis in its original "strong" form is now obsolete, while its various modified weaker variants have in essence become variants of a view of "recent origin" combined with archaic admixture.[70] Stringer (2014) distinguishes the original or "classic" Multiregional model as having existed from 1984 (its formulation) until 2003, to a "weak" post-2003 variant that has "shifted close to that of the Assimilation Model".[71][72]

Genetics[edit]

In the 1980s Allan Wilson together with Rebecca L. Cann and Mark Stoneking worked on genetic dating of the matrilineal most recent common ancestor of modern human populations (dubbed "Mitochondrial Eve"). In his efforts to identify informative genetic markers for tracking human evolutionary history, he started to focus on mitochondrial DNA (mtDNA), passed from mother to child. This DNA material is important because it mutates quickly, thus making it easy to plot changes over relatively short time spans. With his discovery that human mtDNA is genetically much less diverse than chimpanzee mtDNA, Wilson concluded that modern human populations had diverged recently from a single population while older human species such as Neandertals and Homo erectus had become extinct.[73]

With the advent of archaeogenetics in the 1990s, the dating of mitochondrial and Y-chromosomal haplogroups became possible with some confidence. By 1999, estimates cited ranged around 150,000 years for the mt-MRCA, and 60,000 to 70,000 years for the migration out of Africa.[74]

During 2000–2003, there was an active controversy surrounding the mitochondrial DNA of "Mungo Man 3" (LM3) and its possible bearing on the multiregional hypothesis. LM3 was found to have more than the expected number of sequence differences when compared to modern human DNA (CRS).[75] Comparison of the mitochondrial DNA with that of ancient and modern aborigines led to the conclusion that Mungo Man fell outside the range of genetic variation seen in Aboriginal Australians, and was used to support the multiregional origin hypothesis. A reanalysis on LM3 and other ancient specimens from the area published in 2016 showed it to be akin to modern Aboriginal Australian sequences, inconsistent with the results of the earlier study.[76]

See also[edit]

References[edit]

Informational notes

  1. ^ Kay Young McChesney: "Wells (2003) divided the descendants of men who left Africa into a genealogical tree with 11 lineages. Each genetic marker represents a single-point mutation (SNP) at a specific place in the genome. First, genetic evidence suggests that a small band with the marker M168 migrated out of Africa along the coasts of the Arabian Peninsula and India, through Indonesia, and reached Australia very early, between 60,000 and 50,000 years ago. This very early migration into Australia is also supported by Rasmussen et al. (2011). Second, a group bearing the marker M89 moved out of northeastern Africa into the Middle East 45,000 years ago. From there, the M89 group split into two groups. One group that developed the marker M9 went into Asia about 40,000 years ago. The Asian (M9) group split three ways: into Central Asia (M45), 35,000 years ago; into India (M20), 30,000 years ago; and into China (M122), 10,000 years ago. The Central Asian (M45) group split into two groups: toward Europe (M173), 30,000 years ago and toward Siberia (M242), 20,000 years ago. Finally, the Siberian group (M242) went on to populate North and South America (M3), about 10,000 years ago.[43]

Citations

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Bibliography

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